1. Field of the Invention
A technique commonly employed in the art for tempering glass sheets, especially when the sheets are to be bent and then tempered, is to vertically hang each glass sheet from tongs which grip the upper marginal edge portion of the glass sheet, whereby the glass sheet may be conveyed while thus suspended through heating, bending, and tempering steps. Tongs of this type are disclosed in U.S. Pat. No. 3,089,727 to W. J. Hay. The heating step entails suspending the glass sheets within a heating chamber until the temperature of the glass approaches its softening point, and then each sheet in series is conveyed out of the heating chamber and (optionally) into a bending station. A typical vertical bending operation is disclosed in U.S. Pat. No. 3,367,764 to S. L. Seymour, wherein a heat-softened glass sheet is bent by being pressed between a pair of complementary, horizontally reciprocated bending molds. After bending, with the glass sheet still at an elevated temperature, the sheet is conveyed into a tempering station where it is rapidly cooled by applying blasts of tempering medium against the opposite major surfaces of the hot glass sheet so as to establish compressive stresses in the surface portions of the sheet, thereby strengthening the sheet. The tempering medium is usually air, but as used herein, the term may encompass any fluid capable of cooling a hot glass sheet. Such a process has proved to be an economical, high speed method for mass-producing tempered glass sheets, especially bent and tempered automotive glazing and the like.
Apparatus for tempering glass sheets suspended from tongs usually comprises a source of pressurized air which supplies air under pressure through a main supply pipe to a pair of branch conduits. The latter are as identical to one another as possible and each leads to a plenum chamber or reservoir for supplying pressurized air to a series of quench modules that faces one or the other major surface of the glass sheet. A slight difference in pressure is needed to quench shaped glass sheets. In the past, gate valves have been installed in the branch conduits to provide individual pressure control for tempering medium applied to each side of the glass sheet so as to equalize the net force of impingement on the convex and concave surfaces of the bent glass sheet or to overcome any imbalance of force of air supplied to the opposite surfaces that may develop.
One difficulty encountered with tempering glass sheets freely hung from tongs is that directing blasts of tempering medium onto the glass sheets can sometimes cause the glass sheets to sway from side to side. Previous measures such as installing gate valves in the branch conduits, taken to equalize the force of the impingement on both sides of the glass sheet, have not been completely successful because it is virtually impossible to precisely duplicate flow conditions on both sides of a glass sheet, particularly if it is bent with prior art glass sheet tempering apparatus. As a result, sharp side-to-side oscillations of the glass sheet and the tongs are induced.
This buffeting of the glass sheets impedes uniform application of the tempering medium onto the glass sheets, which in turn leads to imbalanced stresses in the tempered product. Such uneven stresses can result in the tempered glass sheet failing to meet strength specifications and may even cause glass breakage during processing. The problem of buffeting is especially troublesome with thin glass (i.e., glass about 4 millimeters or less in thickness), the demand for which has been increasing for use in automobiles. Not only is thin glass lighter in weight and thus more susceptible to buffeting, but also the need for faster rates of cooling to temper thin glass entails the use of higher pressure blasts of tempering medium, which additionally increases the amount of buffeting.
When tempering medium is applied against the opposite glass sheet surfaces, it is also important to control the relative flow rates against the opposite surfaces as well as the absolute flow rate of tempering medium against each glass sheet surface regardless of the orientation plane in which the glass sheet is supported during quenching.
2. Description of the Pertinent Prior Art
Efforts to reduce buffeting in the prior art have included the use of guide wires extending through a quenching apparatus as shown in U.S. Pat. No. 4,006,002 to Hetman, but such an approach has not been found adequate to stabilize glass sheets in the quench to the extent desired.
A further improvement in controlling buffeting is found in U.S. Pat. No. 4,119,423 to Seymour, which provides lateral restraining means to control side-to-side swaying of the tongs and optional glass sheet restraining means in the form of thin, taut guide wires.
In other glass sheet quenching operations, glass sheets are supported in a substantially horizontal plane during quenching. These horizontal supports may include finger supports for the glass sheet as in U.S. Pat. No. 3,455,672 to Dompkowski or roller supports as in U.S. Pat. No. 3,223,498 to Davidson or ring-like supports as in U.S. Pat. No. 3,846,104 to Seymour. In other glass sheet quenching operations, glass sheets are supported in an oblique plane as in U.S. Pat. No. 2,608,030 to Jendrisak.
In all of these quenching operations, it is important to control the flow rate of tempering medium applied against the opposite glass sheet surfaces. In the past, massive control valves such as gate valves, have been used to throttle the flow of tempering medium against each side of the glass sheet during quenching. Such gross control is not fine enough to provide the necessary control of buffeting and also leaves other factors in tempering glass sheets that can be improved.